Today, in the production of metal objects, such as a metal sheets and metal strips, it is common to produce objects having a plurality of layers of different metals or metal alloys. An object of a metal alloy can, for example, be protected with a thin film or a layer of another alloy than the object. For example, an aluminum sheet of a certain alloy can be coated with a surface layer of another alloy having good resistance against corrosion. When producing such objects, the production starts from two thicker metal sheets, which are put together and the stacked metal sheets are then rolled to a desired thickness in a rolling mill. A problem with this production method is that the different metal sheets may have different floating properties during deformation and accordingly the sheets are differently compressed during the rolling. Accordingly, producers of such metal sheets have a need of measuring the thickness of the metal layer. Further, there is an interest in measuring the electrical resistivity of the layer in order to understand changes of the material that happen during the rolling process.
The rolling mill includes at least two rolls and a thickness control system that controls the gap between the rolls and hence the thickness of the produced object. For control of the thickness, the thickness of the object is measured at, at least, one point on the strip after rolling, i.e. after the object has passed through the rolls. This measurement is used as input to the thickness control, together with a desired value of the thickness of the object.